P0116 Code: Subaru Impreza (2017-2023) - Causes, Symptoms & Fixes

Comprehensive diagnostic guide for P0116 on 2017-2023 Subaru Impreza

Important data note

• No NHTSA owner complaints found for this make/model/year/issue combination.
• No recalls found in the NHTSA database for this make/model/year.
• No specific TSBs are cited in the provided data.
• Therefore, this guide uses general automotive knowledge for the P0116 diagnosis, plus the caveat that no complaints/recalls are documented in the supplied data.

CODE MEANING AND SEVERITY

• Code: P0116
• Full name: Engine Coolant Temperature Sensor Circuit Range/Performance
• What it means: The PCM detected the engine coolant temperature (ECT) sensor signal is not within the expected range or the circuit is not performing correctly. This can be caused by a faulty sensor, wiring/connectors, ground/5V reference issues, or, less commonly, an ECM/PCM fault.
• Typical impact on the Impreza: The engine control computer relies on an accurate coolant temperature reading to manage fuel trim, ignition timing (in some cases), and overall warm-up behavior. An out-of-range reading can cause rough idle, poor fuel economy, hard starting when cold, or abnormal enrichment/delay in warm-up. If the actual coolant temperature were extreme but the sensor is reporting incorrectly, the driver may experience overheating or overheating protection events; however, most P0116 cases are sensor or wiring issues rather than an actual catastrophic coolant condition.
• Severity: Moderate. Left unaddressed, it can lead to inefficient fueling, rough running, and potential catalyst/emissions issues. If paired with actual overheating due to a cooling-system fault, it could pose a risk to engine health. Investigate and repair promptly.

COMMON CAUSES ON SUBARU IMPREZA

• Faulty ECT sensor itself (most common cause)
• Damaged, corroded, or disconnected ECT sensor wiring or connector (including poor grounds)
• Problems with the 5V reference circuit or PCM ground to the ECT sensor
• Coolant level issues, air in the cooling system, or coolant contamination affecting readings
• Thermostat issues (sticking open/partially closed) that lead to abnormal actual temps and misleading sensor readings
• Cooling system leaks or low coolant pressure causing readings to drift
• Less common: PCM/ECU fault (usually after other circuit checks have ruled out sensor/wiring)

SYMPTOMS (typical for P0116 on Impreza)

• Check Engine Light (CEL) illuminated with code P0116
• Irregular or delayed thermostat behavior: long warm-up, or gauge readings that don’t match actual engine temperature
• Poor or fluctuating fuel economy due to incorrect fuel trim
• Rough idle or mild to moderate misfire symptoms during cold start or warm-up
• Engine may run slightly rich or lean depending on fuel trim response
• In some cases, no obvious symptoms other than the CEL
• Occasionally, the gauge may show unexpected temperature readings (rapid swings or staying far from normal range)

DIAGNOSTIC STEPS (structured workflow)

Note: Always observe safety precautions with a hot engine and cooling system.

Step A – Validate the code and conditions

• Use an OBD-II scanner to confirm P0116 and note freeze-frame data (engine temp, coolant temp, engine rpm, load, etc.).
• Verify no other codes that could influence engine cooling (e.g., P0115, P0117, P0118, P0128).

Step B – Inspect cooling system basics

• Check coolant level and condition; top off with the correct Subaru-approved coolant if low.
• Inspect for leaks, especially around the thermostat housing, water pump, radiator, hoses, and radiator cap.
• Check for air in the cooling system and perform any necessary bleed/flush per Subaru service procedure.

Step C – Inspect ECT sensor and wiring

• Locate the ECT sensor (usually near the thermostat housing on Subarus; refer to service manual for exact location by engine code).
• Visually inspect the sensor, its connector, and wiring harness for corrosion, cracks, wear, or moisture.
• Disconnect and inspect the connector pins for corrosion or bent pins; reseat firmly.
• Check for damaged insulation or pin backside corrosion that could cause intermittent contact.

Step D – Electrical tests (sensor and circuit)

• With the engine at ambient temperature, check the ECT sensor resistance with a multimeter (ohms) and compare against the service data/spec for the exact sensor used on your engine.
• Check the PCM 5V reference to the sensor and the sensor ground. Key checks:
  • 5V reference wire: should read approximately 5.0 volts with the ignition on (engine off or at rest, depending on procedure).
  • Signal wire: should change resistance/voltage with temperature; if no response or constant, suspect sensor or wiring issue.
  • Ground: ensure a solid ground under the sensor connector and at the ECU ground point.
• If you have access to live data via a scan tool, monitor ECT sensor reading vs actual coolant temperature (you can compare with an infrared thermometer reading on the radiator hose or coolant port at the same location). Large discrepancies indicate sensor or circuit faults.

Step E – Compare actual coolant temperature vs sensor reading

• At cold start: actual engine temperature should be near ambient; ECT reading should reflect cold (low value). As engine warms, ECT reading should rise smoothly.
• If readings are erratic, jumpy, or out of sync with actual engine temperature, suspect sensor or wiring.
• If readings are consistently incorrect but the wiring/test checks are clean, replacement of the ECT sensor is warranted.

Step F – Test the thermostat and cooling-system integrity

• If the sensor tests are inconclusive, consider thermostat function: a thermostat stuck closed or stuck open can cause abnormal readings or real overheating/underheating, which can trigger P0116 if the PCM detects sensor inconsistency during operation.
• Consider a coolant flush and thermostat inspection/replacement if the cooling path is suspect or if the temperature behavior is abnormal.

Step G – Rule out PCM or wiring faults

• If the sensor tests and wiring checks are all normal yet P0116 remains, the PCM/ECU could be at fault. This is less common and usually a last step after swapping sensors/wiring. In some cases, a software update (not a recall) or a dealer diagnostic can identify ECU-level issues.

Step H – Clear codes and test drive

• After any sensor or wiring fix, clear codes and perform a road test to confirm the issue is resolved. Monitor live data to ensure ECT readings track with actual coolant temperature.

RELATED CODES

• P0115: Engine Coolant Temperature Sensor Circuit Malfunction
• P0117: Engine Coolant Temperature Sensor Low Input
• P0118: Engine Coolant Temperature Sensor High Input
• P0128: Coolant Thermostat Temperature Below Thermostat Regulation (thermostat-related, can accompany ECT issues)
  Note: These are commonly seen with ECT problems; addressing P0116 may reveal one of these related codes if the issue involves the sensor range or circuit.

REPAIR OPTIONS AND COSTS (2025 prices)

Prices vary by region and shop, but here are typical ranges for common repairs on a Subaru Impreza. All figures are ballpark ranges including parts and labor; diagnostic fees are separate unless waived with repair.

• Diagnostic fee (for the initial assessment): roughly $100–$120 (may be charged by some shops even if you don’t proceed with a repair).

• ECT sensor replacement

  • Part: $15–$60
  • Labor: $40–$120
  • Typical total: $60–$180

• Thermostat replacement (often done if cooling symptoms accompany P0116 or if thermostat is suspected)

  • Part: $15–$60
  • Labor: $150–$300
  • Typical total: $165–$360

• Coolant flush and fill

  • Part: minimal (coolant itself)
  • Labor: $60–$120
  • Typical total: $60–$120

• Wiring harness or connector repair (if wiring to the ECT sensor is damaged)

  • Parts: $10–$100 (depends on extent of harness repair; may require new connectors or pigtails)
  • Labor: $100–$400 (depends on damage and accessibility)
  • Typical total: $110–$500

• PCM/ECU fault (rare)

  • Parts: thousands for a new unit or reprogramming
  • Labor: variable
  • Note: This is an outlier scenario and much less common than sensor/wiring issues.

Practical scenario examples

• Scenario A: P0116 caused by a faulty ECT sensor with clean wiring

  • Parts: ECT sensor $20–$50
  • Labor: 0.5–1.0 hour
  • Total: roughly $60–$170

• Scenario B: P0116 with suspected thermostat issue

  • Parts: ECT sensor + thermostat
  • Labor: 1.5–2.5 hours
  • Total: roughly $220–$520

• Scenario C: P0116 persists after sensor replacement; wiring harness damage found

  • Parts: harness/connectors $20–$120
  • Labor: 2–4 hours (harness repair/soldering)
  • Total: roughly $240–$520 (plus potential diagnostic costs)

DIY vs PROFESSIONAL

• Do-It-Yourself (DIY) options

  • Pros: Lower parts cost, no labor charges, good learning experience.
  • Cons: Cooling system work can be messy and risky (hot coolant), risk of improper reassembly, difficulty diagnosing wiring references without proper tools.
  • DIY steps that are reasonable: visually inspect wiring and connectors; swap in a known-good ECT sensor if you’re comfortable; test sensor resistance with a multimeter; use an OBD-II scanner to monitor live data; perform a coolant level check and basic cooling-system check.
  • Tools you’ll want: multimeter, infrared thermometer or temperature gun (to compare actual temperature), OBD-II scanner with live data, basic hand tools, possibly a coolant system bleed kit.

• Professional options

  • Pros: Accurate electrical tests (5V reference, ground integrity), proper testing of sensor vs actual coolant temperature, correct bleed/fill of cooling system, proper torque on thermostat and sensor, and a definitive determination if PCM is at fault.
  • Cons: Higher labor costs; more time required depending on shop availability.
  • Expect a diagnostic step to include checking sensor, wiring, 5V reference, and then replacement as required.

PREVENTION

• Use the correct coolant and maintain proper concentrations as per Subaru service guidelines. Subarus typically require a specific coolant type; use the manufacturer-recommended coolant and mix ratio.
• Regular cooling-system maintenance: replace coolant at intervals recommended by Subaru, and address any leaks promptly.
• Inspect ECT sensor and wiring every so often, especially after electrical work or in areas with a lot of heat or moisture. Look for damaged insulation, corrosion, or loose connectors.
• Ensure the cooling system is free of air pockets after service; proper bleed procedures help prevent false temperature readings.
• Prevent overheating: fix leaks, replace aging radiators/hoses as needed, and ensure the thermostat operates correctly.
• Software/ECU updates: While there are no recalls in the provided data, some updates may be available through Subaru service; check for any non-recall software updates that address sensor calibration or low-temperature operation, and apply as directed by the dealer if applicable.

Data limitations and caveats

• The provided data indicates no NHTSA complaints and no recalls for this exact vehicle/issue combination, which means there isn't documented incident-level failure data in this dataset to contradict or refine this guide.
• Specific 5V reference, sensor resistance values, and Subaru-engine-specific wiring details are engine-code dependent (e.g., FA20/FB20 variants). Always refer to the service manual or repair database for your exact engine code and model year, since tolerances and wiring harness layouts can vary.
• Repair costs above are approximate ranges typical in 2025 and can vary by region, shop rate, and exact vehicle configuration. Always obtain a written estimate before work.

In summary

• P0116 on a 2017–2023 Subaru Impreza is typically caused by an ECT sensor, its wiring, or the 5V/ground reference in the circuit. It can affect fuel trim and warm-up behavior but is usually manageable with sensor replacement and a clean electrical harness.
• Start with a sensor/wiring inspection and resistance/voltage tests, then replace the sensor if test results indicate an out-of-range condition. Check the cooling system as needed (thermostat, coolant level, leaks).
• If you want to proceed, prioritize sensor and wiring checks first, then thermostat or coolant service if readings point to actual cooling-system issues.
• Given the lack of documented complaints/recalls in the provided data, the guide focuses on standard diagnostic practices and typical repair costs.